How this works. Your phone's raw GPS altitude is measured against the WGS84 ellipsoid, which can be 20–100 m off true sea level depending where you are. This tool takes several GPS samples, weights them by their reported accuracy, and separately looks up the surveyed terrain elevation (an SRTM-based sea-level datum) for your coordinates — showing you both so you can judge which fits what you need. If you're indoors or upstairs, the terrain figure reflects ground level, not your floor.